Thursday, March 25, 2010

The new Headway LiFePO4 pack is now delivering over 30 kWhrs under freeway driving conditions at 60 mph. This morning my wife Alice and I drove 122.5 miles on a charge on relatively level freeway (a couple hundred feet of elevation change). This worked out to 249.8 Wh/mile. The Progressive Automotive X-Prize requires 100 miles of range and an equivalent of 100 mpg. This is defined as MPGe. The conversion used is 32.82 kWhrs = 1 gallon of gasoline. This means that we need to achieve 328 Wh/mi or better to qualify. This is as measured at the wall receptacle, so charging and battery inefficiency must be considered. A couple of days ago, we drove 80.7 miles with a 252.75 Wh/mi reading from on-board instruments, while the kWhr meter on the wall read 24 kWhrs. That works out to 297 Wh/mi. That's an 85% efficiency. After equalizing the cells all night, the meter at the wall read 25 kWhrs or 310 Wh/mi, which is still well within the requirement--all this with an extra 200 lbs of lead ballast and a second passenger.

With 100 more volts and much less sag from the battery pack, the Tango is now much much faster than with the lead-acid batteries that have been used for all events in the past. We haven't run the 1/4 mile with the Headways yet, but look forward to writing that post.

Tuesday, March 9, 2010

Because it was cold and snowing at times, and because this was only the second cycle on the battery, we only used 28 kWhrs out of the 32 kWhrs that the cells are rated for. The Tango used 260 Whrs per mile at an average speed of about 60 mph on highways with about 400 ft of elevation change. The Elithion BMS gave the following status at the end of the trip.

STATUS OF PARALLEL BATT #0: OK

Cell vtg [V]: 3.05 min(# 46) 3.14 avg 3.18 max(# 20)

Total vtg [V]: 314.7

Cell bd temp, all [C]: +23 min(# 14) +29 avg +35 max(# 46)

Cell bd temp, load off [C]: +23 min(# 14) +29 avg +35 max(# 46)

Cell blk res [mOhm]: 25.4 min(# 0) 25.4 avg 25.4 max(# 0)

Total res: 2540 mOhm

Loads on: 0 (Not balancing: avg voltages below min bal vtg)

Cell boards: 100 (# 0 ~ # 99), all reporting

Banks: 12 (# 0 ~ #11), all reporting

This Headway pack has 1,000 cells mounted in parallel groups of 10, and 100 blocks in series. These were then packaged into 12 modules as pictured below.

There is approximately 200 lbs of lead added to the bottom of the battery box to make up for the lighter weight of the lithium cells. Some of this is visible in the photo below.

There are cells available that would allow 50 kWhrs or more to be fitted in the Tango battery box. This would give up to a 200-mile range.